1. Field of the Invention
The present invention relates to an X-ray mask structure used in lithography when fine patterns for large-scale integrated (LSI) circuits, micromachines or the like are printed on wafers or the like by X-ray exposure; an X-ray mask manufacturing method; an X-ray exposure method; and a device manufactured by using the X-ray mask structure.
2. Description of the Related Art
The development of large-scale integrated circuits typified by DRAMs, 4M (Mega) DRAMs (dynamic random access memory) are now at a mass production stage. Since the technology has made a remarkable advance from 16M DRAMs to 64M DRAMs, the minimum line width required for devices has been reduced from a half micron to a quarter micron. In these semiconductor devices, fine patterns are transferred from a mask to a semiconductor substrate by using near infrared light or far infrared light. The reduction of the line width processed by the wavelength of these lights is approaching a limit. Also, deterioration of depth of focus of electron beams as patterns become finer cannot be avoided. Thus, great expectations are placed on the lithographic technology using X-rays because it simultaneously solves the above-described problems.
Hitherto, a large number of studies of the component materials and structure of X-ray mask structures have been presented. FIG. 2 is a sectional view illustrating a commonly used X-ray mask structure. In FIG. 2, reference numeral 21 denotes a silicon frame for holding an X-ray permeable film; reference numeral 22 denotes an X-ray permeable film; and reference numeral 23 denotes an X-ray absorber. Silicon (Si), silicon nitride (SiN), silicon carbide (SiC) or the like is used for the X-ray permeable film. Gold (Au), tantalum (Ta), tungsten (W) or the like is used as an X-ray absorber.
FIG. 3 shows an example of a conventional method of manufacturing X-ray mask structures, in which an Au plated film is used as an X-ray absorber. An X-ray permeable film 32 is formed by CVD (Chemical Vapor Deposition) to a thickness of 1 to 3 .mu.m on an Si substrate 31 having a thickness of 1 to 5 mm (FIG. 3a). Next, the reverse surface thereof is etched by a potassium hydroxide solution or the like, so that a window is opened (FIG. 3b). After a plating electrode 35 is formed on the X-ray permeable film, a resist pattern 34 is formed (FIG. 3c). Au plating of a thickness of 0.7 to 1.0 .mu.m is performed to form an X-ray absorber 33 (FIG. 3d). The resist and the plating electrode are peeled off, thus forming an X-ray mask structure (FIG. 3e).
It is necessary that the X-ray absorber does not deviate from the required position with respect to the X-ray permeable film manufactured in the above-described way. However, since the internal stress within the X-ray absorber cannot be controlled sufficiently hitherto, a warp occurs in the section between an X-ray absorber 43 and an X-ray permeable film 42, causing a positional deviation of the X-ray absorber, as shown in FIG. 4.
Therefore, a method has been used in which heat treatment (an annealing operation) is performed on a gold plated film forming an X-ray absorber, and thus reducing the internal stress of the gold plated film. It has been reported that it is relatively easy to control the stress of a gold plated film and to form a low-stress film [see K. Suzuki, et al., J. Vac. Sci. Technol. B4(1), pp. 221-225, 1986]. It has also been reported that the stress value of a gold plated film can be changed by heat treatment at a temperature between approximately 20.degree., 30.degree. C. and 120.degree., 130.degree. C. (see K. -H, Muller, et al., J. Vac. Sci. Technol. B4, pp. 230-234, 1986, S. Kuniyoshi, et al., SPIE Vol.923, Electron-Beam, X-ray, and Ion-Beam Technology: Submicrometer Lithographies II pp. 188-196, 1988, and 37th Annual Spring Applied Physics Conference Preliminary Manuscript Vol. 2, p. 489).
It is believed that the reason that the stress is changed by heat treatment at temperatures between approximately 20.degree., 30.degree. C. and 120.degree., 130.degree. C. is that H.sub.2, occluded during plating, is released. In addition, it has been reported that the stress of a gold plated film is reduced by aging.
Therefore, the inventors of the present invention attempted to form a gold plated film which has a low stress by virtue of heat treatment or aging. They formed a resist pattern within a square of 30 mm.sup.2 on an X-ray permeable film and plated it with gold. This was heat treated at a temperature of, for example, 70.degree. to 100.degree. C. at which temperature, there is an expectation of low stress; or the film stress was reduced by aging. Thereafter, the positional deviation of the gold plated film was measured by a length measuring SEM (Scanning Electron Microscope).
The results showed that the pattern produced by the above process is relatively satisfactory in terms of the overall positional accuracy, but large positional deviations occurred locally. When, for example, the 30 mm hole was measured at 400 points (20.times.20), most points showed positional deviations of less than 0.03 .mu.m, but some points (approximately 5 to 10% depending upon the experiment) showed large positional deviations of 0.05 to 0.12 .mu.m.
Since X-ray lithography used with X-ray mask structures aims for a resolution of 0.25 .mu.m, a resolution of 0.25 .mu.m and pattern positional accuracy of less than 0.03 .mu.m in a 30 mm hole are required for the X-ray mask structure. Therefore, the aforesaid positional deviation of 0.05 to 0.12 .mu.m in the 30 mm hole is a major problem.
That is, in the prior art and in the teachings of K. -H, Muller, et al., J. Vac. Sci. Technol. B4, pp. 230-234, 1986, S. Kuniyoshi, et al., SPIE Vol. 923, Electron-Beam, X-ray, and Ion-Beam Technology: Submicrometer Lithographies II pp. 188-196, 1988, and 37th Annual Spring Applied Physics Conference Preliminary Manuscript Vol. 2, p. 489, the stress of the plated film on a Si wafer substrate or the like is computed by using the amount of the warp thereof. In other words, the stress of the plated film computed is an average value of the stress of the entire plated film. However, a low stress based on such an average value has no significance in controlling the stress of an X-ray mask. Rather, every region of the plated film must have low stress.